The present invention relates to a device for writing and reading data to and from an optical recording medium. More particularly, the invention relates to a device for use in writing and reading data by means of a beam of radiant energy to and from a moving magneto-optic recording medium.
Known devices which are used for writing and reading data to and from an optical recording medium, hereinafter known as read/write devices, make use of two physical principles. In one such device, the reflection type, the Kerr effect is used; in a second type, the transmission type, the Faraday effect is used.
In devices using the Kerr effect, light is reflected from the optical recording medium in a complicated optical system in which linearly polarized light is transformed into elliptically polarized light. In such devices special optical elements having little phase difference between the P and the S components of the polarized light are required in order to avoid defective data translation and the source of the optical light beam must have a large power output in order to produce even a small output of light through the objective lens.
Optical read/write devices of the transmission type, e.g. those which use the Faraday effect, have been provided with a view to solving the above problems. A schematic diagram of such a device is shown in FIG. 2. As depicted in FIG. 2, data is input to a magnetic controller 116 and to a laser driver 117 at the input terminals marked 120. Magnetic controller 116 provides a signal for energizing magnetic coil 111 to magnetize an optical recording medium 101 in the desired writing direction. Optical recording medium 101 is a disk which is rotated by means of a drive motor 102 so that the recording occurs along a track in the optical recording medium. The radiation produced by a laser diode 103 is collimated in a collimator lens 104 into a beam which is directed by a total reflection prism 108 to an objective lens 109, where it is focused upon optical recording medium 101. The energy in the focused beam spot causes the temperature of optical recording medium 101 to rise near to the Curie point, enabling the magnetization of optical recording medium 101 to be reversed by a magnetic field produced by coil 111 to effect data storage.
To read the stored data, a linearly polarized beam from laser diode 103 is caused to pass through optical recording medium 101 and the plane of polarization of the beam is rotated thereby in dependence on the direction of magnetization of the medium. Subsequently, the beam passes through an analyzer 112 and falls onto a 4-element stripe-type photodiode 201. In photodiode 201, the received light signal is transformed into an electrical signal which is first amplified by a head amplifier 204 and is then divided in a signal-dividing circuit 205 into a data signal which is output from terminal 119 and into servo signals which are fed to a focusing actuator 202 and a tracking actuator 203.
To find the track which contains data desired for reading out, a track access signal is fed in via terminal 122 to drive tracking actuator 203. A support frame 123 which supports laser diode 103, collimator lens 104, reflecting prism 108, objective lens 109, polarizer 112, and sensor 201, is driven by focusing actuator 202 in the optical axial direction (normal to the optical recording medium) and by tracking actuator 203 in a direction transverse to the optical axial direction, and at right angles to the data track (parallel to the plane of the optical recording medium).
The above-described, known, Faraday principle, optical read/write device has the following problems.
First, support frame 123 is very heavy and, when driven in controlled motion in the axial and transverse directions by appropriate focusing and tracking servo signals, has high inertia and long access times.
Second, high tracking accuracy is difficult to obtain because frame 123 is driven in coarse adjustment by tracking actuator 203 in response to the track access signal, and at the same time is moved in fine adjustment by means of the servo signal. To obtain high tracking accuracy in a short period of time, a large magnetic circuit and large driving currents must be used. Otherwise the optical the recording medium must be rotated at a low speed, with a resulting reduction in transfer rate.
Third, head amplifier 204 is used for amplification of both the data signal and servo signal. However, since the data signal is in the MHz range and the servo signal is in KHz range, it is difficult to design a head amplifier which is suitable without significant deterioration of the S/N ratio.